Thermal water treatment device

ABSTRACT

The invention relates to a device for thermally purifying waste water, comprising a tank through which waste water can flow. The inventive device is characterized by having at least one flow guiding means for guiding the waste water inside the tank in an essentially meandering manner, and by having at least one heating means inside the tank for regulating a predetermined temperature.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 10/519,261 filed Aug. 31, 2005, now U.S. Pat. No. 7,445,722, whichis the national phase of PCT/EP03/06589 filed Jun. 23, 2003, whichclaims priority to DE 102 29 103.9 filed Jun. 25, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a device for thermal waste water purificationwith a container through which the waste water flows and a method ofpurifying melamine-containing waste water.

2. Description of the Related Art

In processing technology it is often necessary to cleanse waste waterflows of undesired substances. For this it is known to treat the wastewater thermally.

So that treatment is successful, i.e., that it reaches the requiredquality for introduction into bodies of water, a certain dwell time isrequired at predetermined temperatures.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a device which isparticularly suitable for cleansing waste waters.

The present invention is directed to a device for thermal waste waterpurification. By using at least one flow guide means the waste water isguided in a meandering fashion into the container. This prevents anymixing back. Setting a predetermined temperature is possible through atleast one heating means in the container (hydrolyser).

Advantageously the flow guide means have at least one wall around whichthe waste water flow is directed. It is particularly advantageous if awall with overflow weir and a wall with underflow weir are providedalternately and parallel as the flow guide means. The waste water isthereby directed along a meandering path through the container.

Advantageously a heating means is provided between two flow guide means,more particularly in an area of an uprising flow. The flow is assistedthrough the rising bubbles.

It is thereby advantageous if a heating means has a device through whichsteam flows, more particularly a tube bank. It is also advantageous ifthe heating means has electric heating.

A particularly advantageous development is if the container is formedcylindrical whereby the longitudinal axis is horizontal. In a furtheradvantageous development the container has a collecting pipe fordischarging gases on the top side.

It is also advantageous if at least two devices of this kind areconnected in series.

The problem is also solved for melamine-containing waste water through amethod having the features of claim 9. According to this for anefficient degradation the temperature in the device is set greater than190° C., more particularly in the region of 220° C. to 240° C. Thepressure is advantageously between 30 bar and 100 bar, more particularlybetween 30 bar and 60 bar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram for waste water purification in a melamineplant including a device according to the present invention;

FIG. 2 a is a schematic sectional view along the longitudinal axis of anembodiment of a device according to the invention;

FIG. 2 b is a schematic sectional view across the longitudinal axisalong the plane B-B of FIG. 2 a; and

FIG. 2 c is a schematic sectional view across the longitudinal axisalong the plane C-C of FIG. 2 b.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the method according to the invention and oneembodiment of a device according to the invention will now be explainedwith reference to treating waste water from a melamine plant.Fundamentally the device can be used however for processing other wastewater.

Before going into the structural design of the device the waste waterpurification process will now be described.

The waste water coming from a melamine plant or from a collectingcontainer system contains a mixture of the following substances or theirammonium and sodium salts (typical values)

Ammonia 1-10 g/kg Carbon dioxide 1-18 g/kg Urea with starting andfinishing processes Cyanuric acid 0.1-0.2 g/kg Ammelide 2-6 g/kgAmmeline 4-12 g/kg Melamine 2-6 g/kg Melam 0.01 g/kg CyanmelamineUreidomelamine NaOH 7-17 g/kg Fogt pH value of about 12

The aim is to break down the noxious water content substances of thewaste water in order to obtain CO₂, NH₃ or HCO₃, CO₂ ⁻ and NH₄ ⁺. Aneutral to at least 30° C. coolable, ammonia-free waste water is to beproduced (depending on the introductory conditions). NH₃ and parts ofthe CO₂ are to be used further.

The compound which most heavily precipitates from waste water ismelamine cyanurate, a salt of the cyanuric acid. The cyanuric acid musttherefore be adequately broken down.

When cooling precipitating melamine cyanurate the cooler movesimmediately. If the solid is crystallised out in a crystallisationapparatus this is expensive and the use of the solid becomes difficult.Melamine has mostly still a slightly higher concentration which howeverif the ammonia is satisfactorily separated off, is mostly still notcritical for the introductory conditions as far as the permitted limitvalues are concerned.

All melamine and oxoaminotriazine (OAT) are broken down with water stepby step to ammonia and carbon dioxide. The equilibrium pressure which isset through the solution and has to be maintained in order to preventevaporation depends on the temperature and the ammonia and carbondioxide content of the solution.

In order to obtain an acceptable breakdown speed of the undesiredsubstances in the waste water, the temperature has to be selected above190° C. The higher the temperature is selected the faster the reactionof the breakdown which has a tendency to reduce the size and costs ofthe apparatus. However with rising temperature the equilibrium pressurerises which has a tendency to increase the cost of the apparatus owingto the increasing wall thicknesses. A small part can also be evaporated.Since the steam has a significantly higher ammonia and carbon dioxideconcentration than the liquid the concentration of these substancesdrops in the liquid phase and the equilibrium pressure falls which leadsto a cheaper apparatus. Since the main part of the steam consists ofwater however and upsets the heat balance with a counter flowinlet/outlet heat exchange, the evaporation represents a considerableloss of energy which pushes up the processing costs.

An optimum is sought between these tendencies which lies at 220 to 260°C. reaction temperature and pressure of 30 to 100 bar, more particularly30 bar to 60 bar. The evaporation is then produced from the charge ofthe waste water.

The basic flow chart is shown in FIG. 1. The device according to theinvention for thermal degradation is the hydrolyser R5 through which thewaste water flows. The connection of this apparatus to the melamineplant will now be explained.

The loaded waste water is preheated in the heat exchanger E1 with theresidual heat of the waste water drawn off from the column sump C8. Itis brought by the pump P2 to the hydrolysis pressure, i.e., the pressureat which the thermal degradation is to be operated. In the counter flowheat exchanger E3 it is preheated practically to the reactiontemperature with the heat of the water flowing down from the hydrolyserR5. The heat exchanger E4 serves for starting up the plant and forcompensating the radiation and heat exchange losses. In the heatedhydrolyser R5 the chemical reactions to ammonia and carbon dioxide takeplace at reaction pressure and reaction temperature. The pH valuethereby drops. Reaction heat and steam losses are supplied through theheat register in the hydrolyser R5. The pressure in the hydrolyser R5 isregulated through the relief valve V7. The steam is supplied to thecolumn C8. The filling level in the hydrolyser R5 is regulated throughthe relief valve V6 after the outflow has given off its heat in thecounter flow heat exchanger E3 to the supply current. Thus apart fromthe vapour losses minimal heat losses arise, the same pressure, reactionpressure, prevails on both sides of the heat exchanger, and at the fluidrelief valve V6 there is no resulting partial evaporation.

In the divided column C8, C9 the ammonia is expelled in the column C8.The column C8 is heated at the sump through the heat exchanger E14. Thewaste water is drawn off at the sump of the column through the said heatexchanger E1 with the pump P15 regulated as regards filling level. Thewaste water is neutralised with the carbon dioxide and supplied to thesewer. The steam of the column C8 is supplied to the gas washer C9 atthe bottom. Liquid is drawn off at the sump from C9 through P10. This isdivided flow regulated through V17 into a return for C8 and into areturn for C9 which is sharply cooled through E11. Through the sharpcooling of the return from C9 the entire gas flow can be condensed inC9. A part of the flow supplied by P10 is furthermore circulated outthrough a filling regulation through the pump P12. This flow is aconcentrated aqueous solution of ammonia and carbon dioxide which isworked up in the urea plant. The pressure is kept in the column throughthe valve V13 and inert circulated out. The waste air can be supplied toa washer.

FIGS. 2 a, 2 b, 2 c show an embodiment of the hydrolyser R5. FIG. 2 ashows a diagrammatic sectional view. FIGS. 2 b and 2 c show sectionsalong the lines B-B and C-C respectively.

The hydrolyser R5 can in principle also be formed in several apparatusesarranged in series whereby the main gas proportion arises in the first.

The hydrolyser R5 is a cylindrically disposed apparatus whose ends areclosed by elliptical bases. The interior consists of a system of severalconnected chambers which are formed as flow guide means through thealternately arranged overflow and underflow weirs 1, 2. The weirs ensurean alternating up and down flow of the fluid and thus provide for littleback mixing. The waste water enters at the inlet 10 into the hydrolyserR5 and flows through the apparatus according to the arrow in thedirection indicated and emerges from the apparatus at the outlet 20again.

In the lower part of the chambers on upstream is located a heatingregister 3 as heating means. The bubble formation which starts throughthe heat input thus assists the overall flow.

The heating register 3 consists for example of a pipe coil or tube bank3 a which can be heated with steam. Also other installations, e.g.,screen bases 1 a, 2 a are possible in place of the weirs. The liquidlevel 11 stands between the upper edge of the apparatus and the upperedge of the overflow weir 1 so that on the one hand the overflow isensured and on the other hand a common gas chamber is formed between theupstream and downstream chamber. Each gas chamber is linked into acollecting pipe 21 lying above same which enables gas discharge 22 andbreathing and thus a uniform filling state in all chambers.

Two sectional views are shown in FIGS. 2 b and 2 c along the planes B-Band C-C. FIG. 2 b thereby shows a side view of a flow guide means 2 withan underflow weir. FIG. 2 a shows a flow guide means 1 with an overflowweir.

The invention is not restricted in its design to the aforementionedpreferred embodiments. Rather a number of variations are possible whichmake use of the device according to the invention and the methodaccording to the invention even with basically different designs.

1. A device for thermal waste water purification with a containerthrough which the waste water flows, the device comprising: at least twoflow guide means which, alternately in parallel as overflow weir and asunderflow weir, are arranged to form a chamber for a meander-shapedguide of the waste water in the container; and at least one heatingmeans, which is arranged between two flow guide means only in a lowerpart of the chamber of the container at the beginning of a rising flowto provide rising bubbles to assist rising flow in the chamber; whereinthe device is configured to produce a supported flow in the chamber andto set to a predetermined temperature.
 2. The device according to claim1, wherein the flow guide means is formed by a wall, around which thewaste water is directed.
 3. The device according to claim 2, wherein thewall is formed by a screen base.
 4. The device according to claim 1,wherein the heating means has a device through which steam flows.
 5. Thedevice according to claim 4, wherein the steam flows through a tubebank.
 6. The device according to claim 1, wherein the heating means haselectric heating.
 7. The device according to claim 1, wherein thecontainer is cylindrical, whereby the longitudinal axis is horizontal.8. The device according to claim 1, wherein the container has, on a topside, a collecting pipe for discharging gases.
 9. A device for thermalwaste water purification, wherein at least two devices according toclaim 1 are connected in series.